Membranes are being applied to treat numerous water problems across the world. Membranes are utilized to remove impurities from solutions and have the ability to remove both dissolved and suspended matter. Membranes are deployed in a variety of pore sizes and in a wide variety of different materials.
The primary limiting factor to membrane applications is the waste stream generated by the separation process that may be a high percentage of the feed stream to the system. Disposal of the waste stream can be costly and in many applications there is simply nowhere for the waste stream to go. In these applications, a zero liquid discharge system is required. Typically, thermal means are used to further concentrate and eventually dry the waste stream at a high cost. Increasing the recovery of membrane systems would therefore provide substantial cost savings and allows membrane applications that would otherwise be impossible.
Membranes that have the ability to remove dissolved inorganic matter are typically referred to as hyperfiltration membranes or reverse osmosis membranes. However, many specialized formulations of membrane material have been created that exhibit varied rejection characteristics. These membranes have been described as nanofiltration membranes, low-pressure membranes, low-rejection membranes, energy saving membranes, and the like. For the purposes of this disclosure, the use of any membrane that rejects dissolved inorganic material will be considered reverse osmosis.
Reverse osmosis, as the name suggests, is the opposite of osmosis. Solutions have an osmotic pressure that is determined by the amount of dissolved matter in the solution. The osmotic pressure of a solution must be overcome for reverse osmosis to take place. Therefore, the ability to concentrate a solution by reverse osmosis is limited by the pressure that can be applied to the membrane. Currently, standard, commercially-available reverse osmosis membranes have a maximum operating pressure of 1200 PSI. However, there are custom membranes in standard configurations that have operating pressures up to 1500 PSI. The maximum operating pressure of the membrane will determine the maximum concentration attainable by reverse osmosis for any solution.
The maximum recovery of a reverse osmosis system is achieved at the maximum concentration of the concentrate stream. If the osmotic pressure limit is attained in the concentrate stream, no further water will pass through the membrane and the maximum attainable recovery is achieved. However, virtually all solutions that are processed by reverse osmosis will foul the membranes at concentrations much lower than the osmotic pressure limit.
There are several categories of contaminants that can foul membranes, including: particulate matter, organisms that multiply within the membrane system, compounds that tend to adhere to the membrane surface, and dissolved contaminants that exceed their solubility and precipitate as they are concentrated in the membrane system. Particulate matter is present in virtually all water sources and it is therefore common practice to filter water through cartridge type filters with a pore size of 1-5 microns prior to processing by reverse osmosis. This prevents large particles that can plug the feed spacer from entering membranes. The particles that are smaller than the prefilter pore size are swept through the membranes by the cross flow. At standard recoveries of 75-90%, the small particles typically do not cause fouling problems. At recoveries exceeding 90%, particulate fouling can be problematic. Uncontrolled growth of organisms in membrane systems will cause membrane fouling. This will occur regardless of the recovery rate. There are numerous compounds that tend to adhere to membranes. They are typically organic compounds. Some compounds can be removed by cleaning agents and some are considered permanent. The fouling by these compounds is primarily a function of overall throughput of fluid through the system and the concentration in the feed stream of these contaminants. The recovery of the system has a marginal effect. Dissolved contaminants that exceed their solubility as they are concentrated in the membrane system are very common. These include the carbonate, sulfate, and fluoride salts of calcium, magnesium, strontium, and barium and also silica. These sparingly-soluble compounds typically define the recovery limit in reverse osmosis systems.
Several methodologies have been utilized to address the membrane fouling associated with sparingly-soluble compounds. For many years it was common to utilize cation exchange water softeners as pretreatment to reverse osmosis. This practice has subsided due to the high cost associated with installing and regenerating water softeners, especially for water sources where the cation exchange load is high, as is common in many ground water supplies.
U.S. Pat. No. 6,537,456 describes a process utilizing weak acid cation exchange prior to reverse osmosis. This process is plagued by the same problem as other cation exchange processes in that there is an ion-for-ion exchange between the resin bed and the water. Upon exhaustion of the bed, it must be regenerated and the regenerant chemical has significant cost. Furthermore, transport of hazardous chemicals such as acids represents a logistical problem in large reverse osmosis applications.
Numerous inventions have been disclosed that purportedly inhibit scale deposition on reverse osmosis membranes by use of magnetic or electrical fields. U.S. Pat. Nos. 6,936,172, 6,217,773 and 6,651,383 all pertain to controlling scale formation by magnetic or electrical fields. However, there has not been commercial application of this technology due to unpredictable performance and high cost. Scale inhibiting compounds have been developed that prohibit crystal formation and can prevent scale from attaching to the membrane surface. This is the most prevalent method of controlling precipitation of sparingly-soluble compounds in commercial, industrial, and municipal reverse osmosis applications.
An important aspect of membrane system design is maintenance of adequate cross flow velocity at the surface of the membrane. This is necessary to prevent deposition of particulate mater on the membrane surface, to minimize the zone of high concentration of dissolved matter at the membrane surface as permeate diffuses across the membrane and contaminants are left behind, and to allow scale-inhibiting chemicals to have adequate contact with compounds over their solubility level.
Commercial, industrial, and municipal membrane applications involve multiple individual membrane elements. These are placed in pressure vessels and arranged in an appropriate manner to provide adequate cross flow. Each individual element is limited to approximately 15% recovery. However, placing the elements in arrays of multiple elements allows higher recovery through the entire system. Membrane element manufacturers provide software to the designers of membrane systems that computes the flow, pressure, and recovery values throughout the chosen membrane array. This data is used to determine the optimum arrangement of elements in a system to attain the desired flow and recovery. Systems are typically constructed of multiple vessels with each vessel containing six elements. The concentrate from the first element becomes the feed to the second vessel and so forth, comprising a series of six elements. The permeate flow from all the elements is connected in a common center tube. These vessels are arranged in banks of vessels in parallel. When the concentrate from one bank becomes the feed to a smaller number of vessels in a succeeding bank, the succeeding bank is referred to as another stage of membranes. Seawater systems typically consist of a single stage and are operated at a relatively low recovery of approximately 35%. The most common array of membranes for brackish water consists of two stages in which the first stage has approximately twice as many membrane vessels as the second stage. Such a system has a typical overall recovery of approximately 75% while maintaining adequate flow velocity to each membrane. It is possible to maintain adequate flow velocities while increasing overall recovery by adding another stage with approximately half as many membranes as the second stage for an overall recovery of approximately 90%. However, it is common for sparingly-soluble compounds to reach maximum saturation capability with scale inhibitor technology at this level of recovery.
Manufacturers of scale inhibitors provide software to the designers of membrane systems that evaluate solubility of various sparingly-soluble compounds at various recovery rates, scale inhibitor dosage, and influent water characteristics. By using the software provided by membrane manufacturers and scale inhibitor manufacturers, it is possible to accurately predict the maximum recovery design for a reverse osmosis system at any given flow rate and water quality.
Unfortunately, the predicted recovery based on these designs are too low to make these processes feasible or too costly to realistically operate under typical field conditions. Therefore, there is a need for alternative system designs that substantially increase recovery beyond the determined maximum from the manufacturer's software for any given water supply. This would preferably be accomplished by treating the concentrate to remove sparingly-soluble compounds and allow further concentration by additional stages of reverse osmosis.
There are patents and published patent applications describing electrolytic treatment of aqueous solutions to remove ions, and/or prevent fouling, biofouling or scaling. For example, US Patent Application Publication No. 2006/0060532 discloses a process in which concentrate from a sea water reverse osmosis system is subjected to electrodialysis, utilizing charged membranes to remove or exchange certain salts. U.S. Pat. No. 6,217,773 discloses a method of placing an electrical field around a RO membrane to reduce fouling. There are several US patent applications that disclose methods of reducing biogrowth or disinfecting water with electrical devices including U.S. Pat. Application Publication Nos. 2005/0230312 and 2005/0087484. U.S. Pat. No. 6,783,687 and US Pat. Application Publication No. 2004/0007452 both describe a method of desalting water utilizing an electrical field to separate ions from water.
U.S. Pat. No. 5,501,798 discloses a process for attaining higher recovery from reverse osmosis by removing sparingly-soluble compounds from reverse osmosis concentrate. This process utilizes pH adjustment and/or introduction of seed crystals to the concentrate water to precipitate sparingly-soluble compounds which are then filtered from the concentrate solution. The solution is then fed back into the inlet of the single reverse osmosis processor. There are several issues that make this design impractical. First, while adjusting the pH to the precipitation point of sparingly-soluble compounds such as calcium carbonate is an obvious method described in fundamental chemistry texts, it is impractical from a cost and materials handling standpoint for commercial application. The pH would also have to be adjusted back down following the precipitation of these compounds, requiring copious amounts of both caustic and acid. Additionally, seed crystals have been shown experimentally to have limited ability to precipitate compounds from reverse osmosis concentrate. Further, these seed crystals are specific to particular compounds requiring adaptation of this method for each sparingly-soluble compound that may contaminate the fluids being purified. Also, return of the concentrate to the inlet of the reverse osmosis processor limits application of the process to smaller, batch-type applications. Therefore, this method of removing sparingly-soluble compounds from reverse osmosis concentrate is not effective at a cost or volume useful to purify contaminated fluids.
Thus, there is a need for a fluid processing procedure that will economically prevent or reduce fouling of the membranes of a reverse osmosis unit to allow high throughput at or close to the osmotic pressure limit.